June 24, 2011

A recent study has attempted to use a long-term (~2,100 years), local (coastal North Carolina), determination of sea level derived from the build-up of salt marsh sediments to better characterize the behavior of global sea level (and by proxy, global temperatures) over the same multi-millennial time period. Based upon the results of this investigation, the research team led by Andrew Kemp from the University of Pennsylvania, concludes that there were four rather distinct periods of sea level rise over the past 2,100 years. Here is how they describe the first three:

Sea level was stable from at least BC 100 until AD 950. Sea level then increased for 400 y at a rate of 0.6 mm/y, followed by a further period of stable, or slightly falling, sea level that persisted until the late 19th century.

Since then, sea level has risen at an average rate of 2.1 mm/y, representing the steepest century-scale increase of the past two millennia. This rate was initiated between AD 1865 and 1892. Using an extended semiempirical modeling approach, we show that these sea-level changes are consistent with global temperature for at least the past millennium.

But can a paleo-record of sea level rise from basically one locality (e.g., coastal North Carolina) provide a good indication of the long-term history of global sea level rise? Obviously, the authors think so, but others are not so sure.

Following the publication of the Kemp paper in the Proceedings for the National Academy of Sciences (a journal with rather questionable peer-review procedures), the German magazine Der Spiegel ran an article which reflected some of the skepticism of the methods/results. The article begins:

After reconstructing sea level patterns over the last 2,000 years for the first time ever, researchers have found that the dawn of the industrial age initiated an unprecedented rise in waters. But critics complain the study is too narrow.

Der Spiegel goes on to explain the problem:

But other experts are doubtful. They see the new study’s limited data as its primary drawback, and question whether something based on findings from the North Carolina coast alone can be applicable for the whole world. “This study is, therefore, not at all suited for making predictions,” says Jens Schröter of the Alfred Wegener Insitute for Polar and Marine Research in Bremerhaven, Germany.

But the study’s authors, among them Stephan Rahmstorf, attempt to reassure us that the findings are robust:

Rahmstorf and his colleagues concede that local sea level fluctuations can differ from global trends. Nevertheless, the scientists conclude that their data, by and large, reflects the changes in global sea levels.

But do they really?

A paper published a few years ago by Alexander Kolker and Sultan Hameed “Meteorologically driven trends in sea level rise” (and covered by World Climate Report) seems germane.

In this paper, these researchers investigated whether the variability and trends in sea level rise along the North American and European coasts of the Atlantic Ocean could be at least partially explained by variations in the strength and position of the dominant atmospheric circulation patterns that characterize the Atlantic region.

And not only did they find that they were, but to quite a sizeable degree. In fact, they found that nearly 70% of the observed long-term (~century) trend in the five tide gauge locations in their study could be explained by meteorological variability (as opposed to say, anthropogenically-forced climate change). For instance, at the location (Charleston, SC) that was closest to the Kemp et al. study sites in North Carolina, Kolker and Hameed determined that meteorological variability could explain 73% of the observed trend in sea level rise for the period 1920-2003.

So while Kemp et al. assume that the influence of “natural climate modes” is only large “over short time scales” and “become progressively smaller as longer time scales are considered” (an assumption which allows them to link sea level rise changes in North Carolina to global temperatures), Kolker and Hameed find that for time periods at least as long as a century (the full length of the period that they studied) that meteorological variability (i.e., the influence of natural climate modes) can dominate the sea level rise signal. In fact, Kolker and Hameed find that meteorological variability has acted to more than double the rate of sea level rise caused by other factors (of which “global warming” is one potential factor—Kolker and Hameed list several other including “sediment compaction” and “volcanically induced changes in ocean heat content”).

All which goes to show that the sea level rise from a single location may not be that representative of global behavior—supporting the critical comments in Der Spiegel.

Has sea level risen over the past two millennia? Almost certainly. Is the rate of global sea level rise tied to global temperatures? Yes. Will future global warming lead to rising seas? Yes again. But, as with all other questions about global warming and its impacts, the important questions are not really whether it is happening, but rather how great are the impacts. The rate of global warming is rather pokey. So too is the resultant sea level rise (recall that a recent study did not find an acceleration of the rate of sea level rise over the 20th century, and that satellite observations show that the short-term rate of sea level rise is declining; for more details, see our recent WCR article “Sea Level Rise: Still Slowing Down“). And as such, natural variability (in all aspects of the climate system) still is a major force to be reckoned with—acting with a magnitude and over timescales probably much greater than it is generally given credit for (at least by the humans-are-responsible-for-everything crowd).

So the take home message here, is that what happens in coastal North Carolina (or any other specific locale) does not well-reflect the world, nor vice versa.

References:

Kemp, A.C., et al., 2011. Climate related sea-level variations over the past two millennia. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1015619108.